Method for building neighboring cells lists in cellular radiocommunications networks

ABSTRACT

A method for defining lists of neighboring cells of a cellular radiocommunications network, includes: obtaining a description of mobility paths followed by users of mobile communications terminals in a geographic area of interest; and including a first cell in the list of neighboring cell of a second cell in case, based on the mobility paths&#39; description, to ascertain that there is a mobility path joining the first and the second cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of telecommunications,particularly radiocommunications, and even more particularly to mobileradiocommunications networks, viz. cellular radiocommunication networks,like mobile telephony networks of the second generation (GSM6—GeneralSystem for Mobile communications—networks) and of the third generation(UMTS—Universal Mobile Telecommunications System—networks).Specifically, the invention concerns a method for building neighboringcells lists to be used by mobile communications terminals to accomplishhand-over from one cell to another.

2. Description of the Related Art

Cellular networks are formed of a plurality of network cells which,altogether, provide radio coverage of a geographic area of interest. Itis possible to ideally subdivide said geographic area into a discretenumber of elementary area elements or “pixels” (e.g., square areaportions of some tens meters of side); each cell is defined as thesubset of pixels that are “served” by the radioelectric signalirradiated by a given transmission station (in the GSM, a BTS—BaseTransceiver Station—or a “Node B” in the UMTS).

A characterizing feature of cellular networks is that they allow usermobility. The passage of a generic user, having a mobile communicationsterminal registered to a cellular network, from one cell of the networkto another is handled by means of a procedure referred to as“hand-over”. If the two cells belong to a same system (i.e., they areboth cells of a GSM or UMTS network), an “intra-system hand-over” issaid to take place; on the contrary, in case of heterogeneous networks(e.g., implementing both the GSM and the UMTS systems), the passage ofthe user from the cell of one system (e.g., a GSM cell, or a UMTS cell)to a cell of another system (a UMTS cell or, respectively, a GSM cell)is called “inter-system hand-over”.

The hand-over procedure is invoked when the level and/or quality of theradio communication channel currently supporting the communications of auser within a cell worsen below a predetermined minimum level, while thelevel of the signal reaching the user terminal and coming from aneighboring cell is sufficient to sustain the communication. In such acase, the network causes the communications with that user to migratefrom the radio channel used so far and belonging to the cell of originto a new radio channel, belonging to the neighboring, destination cell.

In order to successfully accomplish the hand-over, the destination cellneeds to belong to a list of cells which the user mobile terminalexploits, while connected to the cell of origin and engaged in a call,to conduct a signal measurement campaign. When the mobile terminalconnects to a network cell, the network provides to that terminal thelist of neighboring cells (the so-called “NeighBoring Relations or NBRset) that the network knows being neighbors to the cell the terminal isconnected to. The mobile terminal, while engaged in a call andcommunicating with the cell it is currently connected to, measures thelevel of the signals it receives from the other cells in the NBR set,and reports the measurement results to the network, so that the latteris kept constantly informed of when, and towards which destination cell,the hand-over is to be performed. Differently, when in “idle mode”, thecommunication is mainly unidirectional, from the network to theterminal.

The NBR set may include cells of the same system (GSM, UMTS),geometrically adjacent to (i.e., cells having boundaries in common with)the cell the mobile terminal is currently connected to (“intra-systemadjacency relationships”), as well as cells of other systems that arespatially close to the cell the mobile terminal is currently connectedto (“inter-system adjacency relationships”).

The NBR sets for the various network cells are determined in the networkplanning phase, and are then stored in the network apparatuses, ready tobe provided to the terminals of users that connect to the network.

The network planning is usually made with the help of dedicated softwaretools.

Generally, the NBR sets are defined taking into account the topology ofthe network under planning, possibly in combination with electromagneticfield propagation aspects.

Methods for defining the NBR sets in the network planning phase are forexample described in the published European patent application EP 1 427234; essentially, the aim of those methods is to define the NBR set fora generic network cell putting in it the cells that, most likely, willbe destination cells of hand-over procedures from the considered cell,once the network will be deployed.

SUMMARY OF THE INVENTION

The Applicant has observed that the current methods for defining NBRsets are not satisfactory.

For example, a limit of the methods described in the cited document EP 1427 234 is that all the pixels of the area of interest are consideredequally probable in terms of capacity of generating traffic.

In particular, the Applicant has observed that both in the case the NBRsets are built based on the topology of the network under planning(i.e., a cell is included in the NBR set of another cell if the bestserver area of the former cell contacts, i.e., is adjacent to, the bestserver area of the latter cell), and in the case the NBR sets are builtbased on electromagnetic field propagation aspects (i.e., based an thelevel of the signals received in a certain pixel other than the bestserver signal), not all the cells neighborhood relationships thusestablished will in the practice exhibit a significant probability ofgenerating hand-over requests to the network.

On the other hand, the size of the NBR sets is intrinsically ratherlimited: a mobile terminal cannot indeed be asked to perform measurementcampaigns on too high a number of signals; for this reasons, in thepractice NBR sets may include, for example, 32 cells.

Thus, known methods for defining NBR sets may lead to include into theNBR set of a certain cell also cells that, albeit being topologically orelectromagnetically neighbor, would never experience a hand-over,neglecting instead some cells that, on the contrary, would in thepractice be involved in hand-over procedures.

The Applicant has found that the generation of requests of hand-overdepends not only on the topology of the network, nor only on theelectromagnetic field propagation aspects, but also, to a great extent,on the fact that in the area of interest specific user motion pathsexist. The Applicant has therefore found that handover functionalitiesmay be improved by taking into account, when building the NBR sets, thepaths traveled by users in the area of interest, in particular inconnection with the coverage areas of the considered network cells.

Such specific user motion paths may in particular be determined by theurban and extra-urban streets and routes plan (streets, routes,highways, and the like), and by railways.

The Applicant has found that once this additional information is takeninto account in the definition of NBR sets for the cells of the networkunder planning, much better results are obtained, in that it is possibleto exclude from the NBR lists those neighboring cells that are veryunlikely reached in a direct way by users moving in (and connected to) aconsidered cell, i.e. those neighboring cells for which the handoverfrom the considered cell is very unlikely.

The present invention thus relates to a method for defining lists ofneighboring cells of a cellular radiocommunications network, comprising:

-   -   obtaining a description of mobility paths for users of mobile        communications terminals in a geographic area of interest; and    -   including a first cell in the list of neighboring cells of a        second cell in case, based on the mobility paths description, it        is ascertained that there is a mobility path joining the first        and the second cells.

Preferably, the method further comprises:

-   -   dividing the geographic area of interest into a plurality of        elementary area elements; and    -   assigning every elementary area to a respective network cell.

Advantageously, obtaining a description of mobility paths for users ofmobile communications terminals in the geographic area of interestcomprises obtaining at least one vectorial description of said mobilitypaths.

The at least one vectorial description may comprise, for each mobilitypath, at least one vector item formed of a pair of geographic pointsbelonging to the mobility path.

Preferably, the step of including a first cell in the list ofneighboring cell of a second cell in case, based on said mobility pathsdescription, it is ascertained that there is a mobility path joining thefirst and the second cells, comprises including the first cell in thelist of neighboring cells of the second cell if at least one pixel ofthe first cell and at least one pixel of the second cell belong to oneof the vector items.

The first cell is included in the list of neighboring cells of thesecond cell if said at least one pixel of the first cell is adjacent tosaid at least one pixel of the second cell.

The method may further include:

-   -   for each of the user mobility paths, providing a statistical        description of users motion speeds along the mobility paths, and    -   based on said description of users motion speeds, assessing        potential hand-over criticalities.

Preferably, for a user of a mobile communications terminal moving alonga mobility path from the second cell to the first cell, assessingpotential hand-over criticalities comprises checking if a permanencetime of the user in the first cell is lower than a predetermined minimumtime.

The present invention also relates to a method for making handover in acellular radiocommunications network, comprising:

-   -   obtaining a description of mobility paths for users of mobile        communications terminals in a geographic area of interest;    -   including a first cell in a list of neighboring cells of a        second cell in case, based on said mobility paths description,        it is ascertained that there is a mobility path joining the        first and the second cells;    -   making handover when a user of a mobile communications terminal        passes from said second cell to said first cell using said list        of neighboring cells.

The present invention further relates to a computer program comprisinginstructions adapted to implement such method.

Moreover, the present invention relates to a data processing systemadapted to implement such method when programmed to execute the abovecomputer program.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent reading the following detailed description of an embodimentthereof, provided merely by way of non-limitative example, descriptionthat will be conducted making reference to the annexed drawings,wherein:

FIG. 1 schematically shows a small portion of a geographic area intendedto be covered by a cellular network;

FIG. 2 schematically shows a subdivision into elementary area elements,or pixels, of the portion of geographic area of FIG. 1 used in a networkplanning phase, according to an embodiment of the present invention;

FIG. 3 schematically shows the main functional components of a dataprocessing apparatus that, suitably programmed, is adapted to carry outan NBR set planning method according to an embodiment of the invention;

FIG. 4 schematically shows a vector description of a mobility path ofFIG. 1;

FIG. 5 is a schematic flowchart of a method according to an embodimentof the present invention for defining neighboring cells list;

FIG. 6 pictorially shows a criterion for including a cell in theneighboring cells list of another cell;

FIG. 7 is an exemplary diagram of a typical speed along a genericmobility path, used in an embodiment of the present invention; and

FIG. 8 pictorially shows the case of an inter-system hand-over between aUMTS and a GSM cell.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

With reference to the drawings, in FIG. 1 a small portion of ageographic area 100 is schematically depicted, intended to be covered bya cellular network under planning, or where the coverage of an alreadydeployed cellular network is to be improved. The cellular network may inparticular be a second-generation (“2G”) mobile telephony network, e.g.a GSM network, or a third-generation (“3G”) cellular network, like aUMTS network, or a heterogeneous network, supporting both 2G and 3Gstandards. The specific type of cellular network is per-se notlimitative to the present invention.

Schematically depicted in FIG. 1 is a plurality of transceiver stations105-1 to 105-12, to be deployed or already deployed across thegeographic area 100; each transceiver station 105-1 to 105-12 is shownin conjunction with an associated cell, being the portion of thegeographic area 100 which is served by the considered transceiverstation, i.e. the portion of the geographic area 100 where the signalreceived by that transceiver station is the strongest among the signalspossibly received by other transceiver stations; the generic transceiverstation 105-1 to 105-12 is said to be the “best server” in therespective cell. In the drawing, merely for the sake of clarity, thecells are depicted as being all hexagonal and of the same size, howeverin a practical case the cells will in general be different in shapeand/or size, depending on several factors like the power of therespective transceiver stations, and the morphology of the area 100,which affects the propagation of the electromagnetic field.

Also schematically depicted in FIG. 1 are mobility paths 110-1 to 110-5which are present in the geographic area 100 and which can be followedby users of mobile terminals, when the latter move across the geographicarea 100. The mobility paths 110-1 to 110-5 may represent streets of atown or of a village, country roads, highways, railways and the like.

The planning of a cellular network to be deployed in a geographic areaof interest, or the tuning of an already deployed cellular network inorder to better fit the needs of service provisioning in the coveredarea is usually performed with the help of dedicated software tools,running on data processing apparatus.

Referring to FIG. 2, there is schematically depicted a data processingapparatus 200, which, in one embodiment of the present invention, isadapted to be programmed for executing a software tool used for planningthe cellular network (for example in respect of the portion ofgeographic area 100 shown in FIG. 1). The data processing apparatus 200may be a general-purpose computer, like a Personal Computer (PC), aworkstation, a minicomputer, a mainframe, and it may as well include twoor more PCs or workstations networked together.

The general structure of the data processing apparatus 200 isschematically depicted in FIG. 3. The data processing apparatus 200comprises several units that are connected in parallel to a system bus303. In detail, one (possibly more) data processor (Op) 306 controls theoperation of the computer 200; a RAM 309 is directly used as a workingmemory by the microprocessor 306, and a ROM 311 stores the basic codefor a bootstrap of the computer 300. Peripheral units are connected (bymeans of respective interfaces) to a local bus 313. Particularly, massstorage devices comprise a hard disk 315 and a CD-ROM/DVD-ROM drive 317for reading CD-ROMs/DVD-ROMs 319. Moreover, the computer 200 typicallyincludes input devices 321, for example a keyboard and a mouse, andoutput devices 323, such as a display device (monitor) and a printer. ANetwork Interface Card (NIC) 325 is used to connect the computer 200 toa network 327, e.g. a LAN. A bridge unit 329 interfaces the system bus303 with the local bus 313. Each microprocessor 306 and the bridge unit329 can operate as master agents requesting an access to the system bus303 for transmitting information; an arbiter 331 manages the granting ofthe access to the system bus 303.

With reference again to FIG. 2, the planning of a cellular network callsfor ideally subdividing the geographic area of interest into relativelysmall, elementary area elements or pixels px_(ij) (where i and j are twoindexes which take integer values to span the area of interest), eachpixel being an elementary, unit (in the shown example, square) area ofpredefined width, e.g. a 50 m by 50 m square.

In the cellular network planning, the propagation of the electromagneticsignals irradiated by the transceiver stations 105-1 to 105-12 of thenetwork is simulated, and, based on the level of the signals that, as aresult of the simulation, reaches every pixel, the pixels are assigned,in terms of coverage, to respective cells, which are defined the bestservers in the pixels assigned thereto.

A phase of the cellular network planning calls for defining, for eachcell of the area of interest 100, a list of adjacent cells (theso-called NBR set) which may be involved in a hand-over procedure whenthe users move. When the generic mobile terminal connects to a cell ofthe network, the NBR set of that cell is communicated to that terminal,and the terminal uses the NBR set to determine which signals, receivedfrom other cells, to monitor. Thus, when the user is engaged in a calland, due to the movements of the user, the level of the signal receivedfrom the transceiver station of the cell the mobile terminal iscurrently connected worsens too much, the cell to which hand over thecall is readily determined.

According to an embodiment of the present invention, in the definitionof the NBR sets, in addition to using information about the networktopology information, information about the mobility paths of the userswithin the area under planning are exploited.

In particular, in one embodiment of the present invention, a convenientway to describe the mobility paths of the users within a certaingeographic area is using one or more vectorial cartographic layers, eachone representing a respective typology of mobility paths; for example,one vector cartographic layer may represent the plan of the streets of atown, another vector cartographic layer may represent the state routes,still another vectorial cartographic layer may represent the highways,still another vector cartographic layer may represent the railways.

Considering the generic vectorial cartographic layer, a mobility path,like for example a street of a town, may be described by means of a listof “vector items”, wherein each vector item of the list is a couple ofpoints—each point being identified by respective geographiccoordinates—which identify a corresponding segment of the mobility path.For example, referring to FIG. 4, the mobility path 110-1 of FIG. 1 isdepicted alone, in slightly enlarged scale, and consecutive eight pointsP1, P2, P3, P4, P5, P6, P7, P8 are distributed along the path 110-1,each point P1, P2, P3, P4, P5, P6, P7, P8 being described by therespective geographic coordinates (e.g., latitude and longitude). Theeight points P1, P2, P3, P4, P5, P6, P7, P8 define seven vector items(P1,P2), (P2,P3), (P3,P4), (P4,P5), (P5,P6), (P6,P7) and (P7,P8), which,altogether, provide a piecewise-linear representation of the mobilitypath 110-1. The spatial distance between two points of a vector itemgives the resolution of the vectorial layer; as can be seen in FIG. 4,the resolution is lower in correspondence of straight or almost straightsegments of the mobility path, whereas the resolution generallyincreases in correspondence of curves.

Hereinafter, a method of defining NBR sets according to an embodiment ofthe present invention is described, making reference to the schematicflowchart of FIG. 5.

Firstly, based on a description 505 of a current cellular networkconfiguration and a description 510 of the territory of the geographicarea of interest, using an electromagnetic field propagation simulator,the distribution of the electromagnetic field, originated by thecellular network transceiver stations, through the area of interest isestimated (block 515). In particular, as mentioned in the foregoing, thegeographic area of interest is subdivided into elementary area elementsor pixels, and the electromagnetic field in every pixel is estimated.

Based on the estimated field, best server areas are calculated (block520); this involves establishing, for every pixel, which is thestrongest signal received at that pixel, and consequently assigning thepixel to the best server area of the transceiver station that irradiatesthat strongest signal. In particular, considering the case of a GSMnetwork, the signal level considered to establish which is the bestserver is that of the Broadcast Control CHannel (BCCH), which is one ofthe channels defined by the GSM standard; in the case of a UMTS network,the signal level considered may be that of the Common Pilot CHannel(CPICH). As a result, the geographic area of interest is covered byseveral patches (like the exemplary, hexagonal patches depicted inFIG. 1) each patch being a cell of the network.

Then, for every network cell, i.e., for every transceiver station(blocks 525 and 555), the topologically adjacent cells are selected, oneat a time (blocks 530 and 550). By “topologically adjacent” it is meantthat the considered two cells, i.e. the best server areas of therespective transceiver stations, “touches” for at least one pixel. Foreach pair of topologically adjacent cells, it is assessed whether atleast one pair of pixels can be identified, one in the cell underconsideration, the other in the selected topologically adjacent cell,that belongs to one of the mobility paths (in particular to at least anitem of a mobility part) within the geographic area of interest (block535), described in one or more cartographic vectorial layers 540 (forexample, including a vectorial layer of the streets of a town, and avectorial layer of the railways). In the affirmative case (exit branch Yof block 535), a cell that is topologically adjacent to the cellcurrently considered is included in the NBR set of the latter (block545), otherwise it is not included.

In particular, referring to FIG. 6, considering a generic cell Cr, and acell Cs which is topologically adjacent to the cell Cr, the cell Cs isincluded in the NBR set of the cell Cr only if a pair of pixels exists,like the shown pixels px(h,i) and px(j,k), which: a) belong one to thecell Cr and the other to the cell Cs, b) touch each other (i.e., arelocated along the boundaries of the respective cells) and c) belong to avector item (Pm,Pn) of one of the cartographic vector layers 540describing the user mobility paths in the geographic area of interest.

Thanks to the method of the present invention, it is possible to excludefrom the NBR sets being defined those cells that, in the practice, willnever be involved in hand-over procedures, because there are no mobilitypaths that the users can follow to go from one cell to the other. Forexample, referring back to FIG. 1, and considering the cell of thetransceiver station 105-1, it can be appreciated that hand-overprocedures may in the practice involve the cells 105-2, 105-3, 105-5 and105-7, but not the cells 105-4 and 105-6, since there are no mobilitypaths that the users can follow to directly move from cell 105-1 toeither of the cells 105-4 and 105-6.

In a preferred embodiment of the present invention, in addition to adescription of the mobility paths, information about how the userstypically move along the mobility paths in the area under planning isexploited to define the NBR sets. In particular, user mobility modelsare exploited, that are adapted to describe the direction and speed ofmovement of the users along the different mobility paths. For example,in FIG. 7 an exemplary speed profile is shown, describing the (typical)speed of users when moving along the mobility path 110-1 of FIG. 1. Thediagram shows instant speed values varying continuously along the path110-1, however, in an embodiment of the present invention, an averagespeed value may be calculated and assigned to each vector item (P1,P2),(P2,P3), (P3,P4), (P4,P5), (P5,P6), (P6,P7) and (P7,P8). In a simplifiedapproach, an overall average speed value may be calculated for the wholemobility path 110-1, so that every vector item (P1,P2), (P2,P3),(P3,P4), (P4,P5), (P5,P6), (P6,P7) and (P7,P8) of the path is assigned asame average speed.

Exploiting the additional information about the typical speed of usersalong the mobility paths, it is possible to evaluate whether there maybe criticalities in completing hand-over procedures, due to the too hightypical speed of the users, and/or to the size of the best server areas.In particular, according to an embodiment of the present invention, foreach cell adjacency relationship determined in the way described above,it is possible to calculate, based on the information about the typicaluser speed along the mobility paths, a time of permanence of a genericuser in the cell being the destination of a potential hand-over if sucha permanence time is shorter than a predetermined minimum time, thehand-over cannot be successfully completed, and thus it will likelyfail.

The method of the present invention is applicable not only to thedefinition of NBR sets for intra-system hand-over procedures, betweencells of a same system (e.g., GSM or UMTS), but also for the definitionof cells adjacency relationships between cells of different hierarchicallayers of a same system (e.g., the 900 MHz and the 1800 MHz GSM layers),as well as for defining cells adjacency relationships between cells ofdifferent systems, e.g. GSM and UMTS cells, for inter-system hand-overprocedures.

The latter case is schematized in FIG. 8: reference numeral 805 denotesa UMTS cell, and reference numeral 810 denotes a neighboring GSM cell;it is also assumed that mobile terminals moving through said cells aredual-mode.

According to an embodiment of the present invention, in order to declarethat the cell 810 is adjacent, for inter-system hand-over purposes, tothe cell 805, three conditions are to be met.

A first condition is that in at least one of the cartographic vectoriallayers describing the user mobility paths in the geographic area ofinterest, there is at least one vector item that includes a px₁ pixelbelonging to the UMTS cell 805, and a pixel px₂ belonging to the GSMcell 810, and the two pixels “touch” each other, i.e. they have at leastone common side.

A second condition is that a level of the signal-to-noise ratio Ec/Io inthe UMTS cell 805 at the pixel px₁ is lower than a predeterminedthreshold, defining the level of signal-to-noise ratio at which a dualmode, UMTS/GSM mobile terminal on the field will start measuring thesignal level of the GSM network (a so-called “compressed mode” isactivated in the mobile terminal, and the RSSI (Radio Signal StrengthIndicator) of the GSM network starts being measured).

A third condition is that the GSM signal level (the measured RSSI), atthe pixel px₂ should be at least equal to, or higher than apredetermined threshold, defining the signal level at which theinter-system hand-over from the UMTS to the GSM system can start.

In addition, assuming that not only the description of the user mobilitypaths, but also a description of the typical speed of the users alongthe mobility paths is provided, for every pair of UMTS and GSM cellsthat are defined adjacent in the way described above, an indicationabout the timing of the hand-over procedure may be provided. Inparticular, said indication may include, for every pixel satisfying thethird condition listed above, an indication of the time that the typicaluser will spend in the destination GSM cell 810 is provided. This allowsdetecting possible criticalities, due for example to the fact that, dueto the speed of the user, the time lapsed between the activation of thecompressed mode and the instant at which the inter-system hand-over fromthe UMTS to the GSM system starts may be insufficient for completing thehand-over from the UMTS cell 805 to the GSM cell 810.

The present invention provides a method by which it is possible todefine adjacency relationships between the cells of a (homogeneous orheterogeneous) cellular network which are more true-to-reality for thepurposes of managing hand-over procedures, because they take intoaccount the real possibility of motion of the users, thereby avoiding toinclude, in the list of adjacent cells, cells which will never beinvolved in a hand-over, being impossible for the users to follow thosedirections of movement.

The method of the invention can also be used to evaluate an alreadyexisting set of adjacency relationships (irrespective of how they havebeen defined), so as to assess how good it is, from the viewpoint of thehand-over; in particular, the method allows estimating how manyhand-over procedures could possibly fail for missing coverage, and howmany hand-over procedures could instead fail for inadequate timing (dueto the average users speed).

The present invention has been here disclosed making reference to someexemplary embodiments thereof; those skilled in the art will howeverrecognize that several changes to the described embodiments, as well asother embodiments are possible, without departing from the protectionscope defined in the appended claims.

1-11. (canceled)
 12. A method for defining lists of neighboring cells ofa cellular radiocommunications network, comprising: obtaining adescription of mobility paths for users of mobile communicationsterminals in a geographic area of interest; and including a first cellin the list of neighboring cells of a second cell in case, based on saidmobility paths' description, to ascertain that there is a mobility pathjoining the first and the second cells.
 13. The method of claim 12,further comprising: dividing the geographic area of interest into aplurality of elementary area elements; and assigning every elementaryarea to a respective network cell.
 14. The method of claim 13, whereinobtaining a description of mobility paths for users of mobilecommunications terminals in the geographic area of interest comprisesobtaining at least one vectorial description of said mobility paths. 15.The method of claim 14, wherein said at least one vectorial descriptioncomprises, for each mobility path, at least one vector item formed of apair of geographic points belonging to the mobility path.
 16. The methodof claim 15, wherein including a first cell in the list of neighboringcells of a second cell in case, based on said mobility pathsdescription, to ascertain that there is a mobility path joining thefirst and the second cells, comprises including the first cell in thelist of neighboring cells of the second cell if at least one pixel ofthe first cell and at least one pixel of the second cell belong to oneof said vector items.
 17. The method of claim 16, wherein the first cellis in the list of neighboring cells of the second cell if said at leastone pixel of the first cell is adjacent to said at least one pixel ofthe second cell.
 18. The method of claim 16, further comprising: foreach of said user mobility paths, providing a statistical description ofusers' motion speeds along said mobility paths; and based on saiddescription of user motion speeds, assessing potential hand-overcriticalities.
 19. The method of claim 18, wherein, for a user of amobile communications terminal moving along said mobility path from saidsecond cell to said first cell, assessing potential hand-overcriticalities comprising checking if a permanence time of said user insaid first cell is lower than a predetermined minimum time.
 20. A methodfor making handover in a cellular radiocommunications network,comprising: obtaining a description of mobility paths for users ofmobile communications terminals in a geographic area of interest;including a first cell in a list of neighboring cells of a second cellin case, based on said mobility paths' description, to ascertain thatthere is a mobility path joining the first and the second cells; andmaking handover when a user of a mobile communications terminal passesfrom said second cell to said first cell using said list of neighboringcells.
 21. A computer program comprising instructions capable of beingadapted to implement the method according to claim 12, when executed.22. A data processing system capable of being adapted to implement amethod for defining lists of neighboring cells of a cellularradiocommunications network, comprising: obtaining a description ofmobility paths for users of mobile communications terminals in ageographic area of interest; and including a first cell in the list ofneighboring cells of a second cell in case, based on said mobilitypaths' description, to ascertain that there is a mobility path joiningthe first and second cells, when programmed to execute the computerprogram of claim 21.